UV radiation penetrates the ozone layer over two wavelength regimes, UVB (290-320 nm) and UVA (320-400 nm). UVB acts directly on biological molecules, causing the familiar delayed sunburn that arises 12-24 hours after exposure, skin aging, skin cancer (melanoma) and eye photokeratities. UVA acts indirectly with the skin by forming reactive oxygen species, causing an “immediate” sunburn that diminishes within 2 hours after exposure. UVA potentially plays a role in delayed sunburn and skin cancer. Although less energetic, UVA, which accounts for about 5.6% of sunlight, penetrates the skin more deeply, even into the dermis, than does UVB radiation, about 0.5% of sunlight, which generally is limited to the epidermis.
Melanoma has experienced the most rapid increase in the number of cases of all forms of cancer with more than 51,000 cases reported in America each year. As most cases have been attributed to sun exposure, the market for sunscreens presently exceeds $1 billion a year. Sunscreens are substances used to protect the skin by absorbing, reflecting and/or scattering damaging ultraviolet (UV) radiation. Sunscreens are typically used as a component in a cream or lotion. Sunscreen formulations are an article intended for the prevention of a disease and are regulated as an over-the counter (OTC) drug.
In a Final Monograph of May 21, 1999, entitled “Sunscreen Drug Products for Over-The-Counter Human Use” by the U.S. Food and Drug Administration (FDA), conditions were established under which OTC sunscreen drug products are generally recognized as safe and effective and not misbranded as part of FDA's ongoing review of OTC drug products. The Monograph established that an active ingredient of sunscreen products consists of any of the following (within the concentration specified for each ingredient when the finished product provides a minimum SPF value of not less than 2 as measured by a testing procedures established in the Monograph): Aminobenzoic acid (PABA) (15%); Avobenzone (3%); Cinoxate (3%); Dioxybenzone (3%); Homosalate (15%); Menthyl anthranilate (5%); Octocrylene (10%); Octyl methoxycinnamate (7.5%); Octyl salicylate (5%); Oxybenzone (6%); Padimate O (8%); Phenylbenzimidazole sulfonic acid (4%); Sulisobenzone (10%); Titanium dioxide (25%); Trolamine salicylate (12%); and Zinc oxide (25%). Since 1999 Ecamsule (10%) and Phenylbenzimidazole sulfonic acid (4%) have been approved for use as sunscreens. Also approved for use in Europe are: 4-Methylbenzylidene camphor (4%); Bisoctrizole (10%); Bemotrizinol (10%); Bisdisulizole disodium (10%); Drometrizole trisiloxane (15%); Benzophenone-9 (10%); Ethylhexyl triazone (5%); Diethylamino hydroxybenzoyl hexyl benzoate (10%); Iscotrizinol (10%); Polysilicone-15 (10%); and Isoamyl p-Methoxycinnamate (10%).
Of these sunscreens, only Polysilicone-15 is a silicon based polymeric sunscreen with the IUPAC name α-(trimethylsilyl)-ω-(trimethylsilyloxy)poly[oxy(dimethyl)silylene]-co-[oxy-(methyl)(2-{4-[2,2-bis(ethoxycarbonyl)vinyl]phenoxy}-1-methyleneethyl)silylene]-co-[oxy-(methyl)(2-(4-[2,2-bis(ethoxycarbonyevinyl]phenoxy)prop-1-enyl)silylene] with about 55 oxy(dimethyl)silylene units, about 4 oxy(methyl)(2-{4-[2,2-bis(ethoxycarbonyl)vinyl]phenoxy}-1-methyleneethyl) silylene units and about 1 oxy(methyl)(2-(4-[2,2-bis(ethoxycarbonyl)vinyl]phenoxy)prop-1-enyl)silylene units on average with an average molecular weight of 6,070. Statistically the random copolymer should have more than one percent of the chains that have no UV absorbing chromophores. An equivalent polymer of about 1,000 molecular weight would have less than half of the chains containing any UV absorbing chromophores. Polymeric sunscreens have the potential to provide a simple mode of distribution of the sunscreen in a vehicle, and reduce or eliminate absorption of the sunscreen by the skin to which it is applied. Many examples of polymeric sunscreens have been disclosed in the patent literature.
U.S. Pat. Nos. 7,291,322, 6,376,679, 6,312,673, 6,251,373, 6,221,343, 6,214,324, 6,200,557, 6,159,456, and 5,753,209, and U.S. Patent Application Publications 2007/0020204, 2004/0213746, 2002/0054860, and 2001/0026789 are directed to silicone based polymeric sunscreens that are random copolymers similar to Polysilicone-15 with various UV absorbing chromophores. Another form of polymeric sunscreens is random acrylic copolymers, as disclosed in U.S. Pat. Nos. 5,741,924, 5,487,885, 5,099,027 and 4,524,061 (which also discloses the polymers from a cyclic lactams). Substantive polymers are prepared by random vinyl copolymerization are disclosed in U.S. Pat. No. 7,087,692 and U.S. Patent Application Publication 2004/0101498 or by random condensation copolymerization, as disclosed in U.S. Pat. Nos. 4,004,074 and 3,864,473. Water dispersible polymeric sunscreens have been prepared by a random condensation copolymerization with polyethylene glycol monomers, as disclosed in U.S. Pat. Nos. 5,250,652, 5,243,021, and 5,134,223. An acrylamide homopolymer having UV active chromophores at every repeating unit is disclosed in U.S. Pat. No. 4,233,430. U.S. Patent Application Publication 2005/0186152 discloses a polyanhydride modified by the addition of nucleophilic UV active chromophores to place the chromophores on every repeating unit of the polymer chain with the formation of an equal amount of carboxylic acid groups on the polymer chain. U.S. Pat. Nos. 6,962,692, 6,926,887, 6,919,473, 6,899,866, 6,890,521, 6,800,274, and 5,993,789, disclose a homo-polyester sunscreen where UV active chromophores are on every repeating unit of the polymer.
As disclosed polymeric sunscreens have very high levels of UV absorbing chromophores that can result in a non-uniform distribution of the chromophores in the vehicles as in homopolymer or with random copolymers where molecular weights or UV absorbing chromophores are limited to those where a high molecular weights or high levels of UV absorbing units. Hence a polymeric system where the UV absorbing chromophore's distribution through out the polymer can be controlled with every polymer chain having the same proportion of UV absorbing chromophores independent of the molecular weight is attractive for a polymeric sunscreen.